The invention relates to a device for detecting a position of a position indicator which influences a magnetic field generated by an electrical magnetic field source. Additionally, the invention relates to a method for detecting a position of a position indicator using such a device.
When constructing large machines or automobiles or in automated systems and apparatuses, the object of having to measure the position or orientation of parts of the machine or the motor vehicle frequently arises. For reasons of lifetime, reliability and easy mounting, contactless methods and/or magnetic measuring systems, such as, for example, magnetic position indicators, are frequently used since these operate in a contactless manner and, thus, free from wear. A permanent magnet is usually fixed to a movable machine part and a magnetic field sensor fixed at a fixed, i.e. stationary, location. When the magnet moves, this change may be determined using a change in the magnetic field at the location of the sensor.
The temperature dependency of the measurement is of disadvantage in these assemblies since both the magnetic field of the permanent magnet and the sensitivity and offset of the sensor are largely dependent of the temperature. These effects may be attenuated by complicated correction measures. Using 3D measuring methods, as are described, for example, in [1], the dependency on the sensitivity of the sensor and the magnetic field of the permanent magnet may be eliminated systematically. However, the offset of the sensors still has to be corrected in a complicated manner.
Another disadvantage is the dependence on disturbing magnetic fields superimposed on the field of the permanent magnet. These may be either the Earth's magnetic field or strong magnetic fields in industrial plants, such as, for example, aluminum works, or magnetic fields close to electromagnets, for example on scrap yards. However, smaller disturbing fields caused by electrical currents or magnetic materials close to the measuring assembly may also have negative effects on the measuring precision. This may be avoided by a spatially differential (gradient) evaluation of several sensor signals in homogenous disturbing fields. However, for inhomogeneous disturbing fields, the influence by a gradient measurement may only be attenuated, but not eliminated completely.
In modern systems of linear position measurement, two (sometimes even more) 3D magnetic field sensors are mostly used apart from the permanent magnet to achieve independence on temperature and external fields. A temperature-stable position measurement may be achieved by evaluating the direction of the field lines, instead of the magnitude of the magnetic field string. External fields present may be easily suppressed by using gradients so that position measurements relatively safe from disturbances may be achieved, even without magnetic shielding.
An example of the state of elements, available on the market at present, including a permanent magnet as a position indicator and a sensor-integrated circuit (sensor IC) having at least two 3D magnetic field sensors is illustrated in FIG. 6.
FIG. 6 shows a schematic illustration of a device 64 for linear position measurements having 3D magnetic field sensors for recognizing the position of a body 12′ in a permanent magnetic field 16′. The permanent magnetic field 16′ is generated by a permanent magnet 44 arranged at a movable element 32. An evaluating means 18′ comprises five sensors 22a-e configured to detect the permanent magnetic field 16′ in three spatial directions each. The sensors 22a-e are arranged on a board, for example on or in a sensor ASIC, in the shape of a cross. This means that sensors 22a, 22b, 22d, and 22e are each arranged equidistantly in a positive and negative x and z directions around the sensor 22c. Based on the equidistant and defined arrangement of the sensors 22a-e, movement of the mobile element 32 or the permanent magnet 44 may result in variable magnetic field lines or gradients of the permanent magnetic field 16′. The variable orientation of the magnetic field lines or gradients of the permanent magnetic field 16′ may be determined in each of the sensors 22a-e so that a position of the permanent magnet 44 may be concluded based on the defined distances between the sensors 22a-e. A temperature-independent variation of, possibly, improved external fields may be realized already using two, but also more 3D sensors 22a-e, as is described, for example, in [1].
In principle, magnetic fields generated by a permanent magnet and, thus, representing DC magnetic fields may be superimposed by external DC magnetic fields, like the Earth's magnetic field. Suppressing such external magnetic fields may be more complicated in a DC magnetic field which is used for determining positions such that the result may be a deteriorated suppression of disturbing fields. In addition, using permanent magnets for generating the permanent magnetic field 16′ may result in the fact that the permanent magnet 44, when being mounted, for example, to a movable element 32, has to be placed and oriented while keeping position tolerances. Thus, a translatory tolerance may induce only small problems, whereas a rotational tolerance, for example, in particular with a ball, may entail greater problems. In other words, when producing a sensor system, mechanical tolerances arise which may also have an effect on the measuring precision. In many systems, such tolerances are corrected by system calibration. For mechanical mounting, a ball magnet is particularly problematic since the axis of magnetization is not characterized by the geometrical shape of the magnet. In ball magnets, it may consequently be necessitated to specify a magnetic orientation device which orients the magnet correctly relative to the rest of the assembly.
Additionally, permanent magnets may be too expensive, in particular as regards materials becoming ever more expensive, like rare earths.
Linearly variable differential transformers (LVDTS), as are described, for example, in [2], may make use of coil coupling between a primary coil and two secondary coils over a magnetizable core or moveable in one degree of freedom, which is able to move linearly within the coil. A superposition of output voltages of the secondary coils phase-shifted by 180 degrees shows a varying output signal of the LVDT device, depending on the position of the indicator body i.e. the magnetizable core. Ferroinductive displacement transducers, as are, for example, described in [3], exhibit a similar mode of functioning.
Magneto-inductive principles, as are described, for example, in [3], may comprise a coil having a softly magnetizable core. The coil may exhibit a strong change in inductivity when the core is saturated by an indicator magnetic field. With a movement of the position indicator in one degree of freedom, the portion of the saturated material is position-dependent. The movement indicator here is a permanent magnet. Superimposing fields may disturb the measuring principle, which does not allow ideal suppression of disturbing fields.
Inductive analog distance sensors, as are described, for example, in [3], use a change in inductivity of the coil and, thus, a change in the resonant frequency of an oscillating circuit in which the coil is arranged when approximating a magnetizable material, wherein the measuring principle allows movement in one degree of freedom.
Principles, as are used, for example, in sound pickups for guitars, comprise a permanent magnet which magnetizes the strings of the guitar. A coil detects a magnetic field of the strings moved so that a magnetically induced voltage may be transferred to loudspeakers via an amplifier.
A device comprising a constant backbias (permanent) magnet having a magnetizable gear wheel is shown in [4]. Such devices do not allow ideal suppression of disturbing fields.
A device allowing the position of an element to be determined irrespective of external magnetic fields would be desirable.
The object underlying the present invention is providing a device and a method allowing the positions of bodies to be detected in a more robust, space-saving, i.e. small, and cheaper manner.